A bottle or container is formed from a gob of molten glass in an individual section glass container forming machine by first pressing or blowing the gob into a parison and then blowing the parison into the container. When glass is uniformly distributed in this process, wall thickness will be uniform, but when glass distribution is not uniform, thin spots can occur which may result in bottle failure during filling or handling.
The high dielectric constant of glass makes it possible to measure the electrical capacitance of the wall of a glass container located against two elongated electrodes of a sensor which contact the bottle as it is rolled along the sensor. This capacitance increases monotonically as the thickness of the glass container increases and can be measured using an electronic circuit to convert the capacitance to a voltage. A voltage representative of the thickness of the wall can be continuously generated as the bottle rolls along the sensor, and if the voltage decreases below a selected value (the value which represents the minimal wall thickness), the bottle being tested can be rejected.
Most capacitance type glass container wall thickness inspection devices available suffer from accuracy problems due to measuring the small changes in capacitance caused by changes in wall thickness (typically on the order of 10.sup.-3 Picofarad change in measured capacitance per 10.sup.-3 inch change in wall thickness), and the non-linear change in capacitance as a function of thickness over the range provided by glass containers.
The prior art capacitance type wall thickness inspection devices comprise a head assembly attached to a conveyer which is conveying the glass bottles to be inspected, a console which is separate from the head assembly and located some distance from the conveyer and an electrical cable connecting the head assembly and the console. The head assembly contains a measuring oscillator circuit which is analogous to a radio transmitter, with the frequency of the transmission being modulated by the capacitance of the glass wall being inspected. The console contains a reference oscillator circuit which is closely matched to the measuring oscillator circuit. This reference circuit operates on the heterodyne principle and may be thought of as a receiver tuned to the frequency of the measuring oscillator. The frequency difference between the measuring and reference oscillator provides a measure of the wall thickness of the glass containers being inspected. Because the oscillator in the console has to be closely matched to the oscillator in the head assembly, every time a head assembly is replaced, the console oscillator circuit must also be replaced. Generally, a wall thickness inspection machine will include four different data channels, each of which inspects the wall thickness of a different part of the container. To avoid interference, each measuring oscillator in each channel operates at a different frequency. Thus a wall inspection station will have four different reference oscillator assemblies in the console, each matched to one of the four different head oscillator assemblies. Since the oscillator circuits are relatively fragile, glass container inspection machines must keep spare replacement oscillators in stock, otherwise they may have to shut down a production line. Since each head assembly requires a matched console oscillator assembly, the glass manufacturers must generally keep in stock four different sets of head oscillators and matching console oscillator assemblies.